Krummholz or krumholtz formation (German: krumm, "crooked, bent, twisted" and Holz, "wood") — also called knieholz ("knee timber") — is a particular feature of subarctic and subalpinetree line landscapes. Continual exposure to fierce, freezing winds causes vegetation to become stunted and deformed. Under these conditions, trees can only survive where they are sheltered by rock formations or snow cover. As the lower portion of these trees continue to grow, the coverage becomes extremely dense near the ground.[1]

William Rogers Fisher introduced the English terms elfin-tree and elfin-wood to correspond to the German krummholz in his 1903 translation of Andreas Franz Wilhelm Schimper's Plant-geography upon a Physiological Basis (1898).[3]

A variation of this formation is a flag tree or banner tree. Branches on the windward side are killed or deformed by the almost constant strong winds, giving the tree a characteristic flag-like appearance. Where the lower portion of the tree is protected by snow cover or rocks, only the exposed upper portion may have this appearance. This is a rather common occurrence in Red Spruce trees of the highest peaks of the central, even southern Appalachian Mountains, and is most commonly seen in the wind-swept high peaks and plateaus of the Allegheny Mountains. This formation most notably occurs with high frequency in the Dolly Sods and Roaring Plains West Wilderness areas along the Allegheny Front in eastern West Virginia, typically occurring at elevations of 3,800 feet (1,100 m) and higher. Trade winds in tropical regions near the equator can also shape trees in a similar manner.[1]